Quartz Composer uses OpenGL (including GLSL), OpenCL (only in Mac OS X 10.6 and later), Core Image, Core Video, JavaScript, and other technologies to create an API and a developer tool around a simple visual programming paradigm. Apple has embedded Quartz technologies deeply into the operating system. Compositions created in Quartz Composer can be played standalone in any QuickTime-aware application[1] (although only on Mac OS X 10.4 and later), as a system Screen Saver,[2] as an iTunes Visualizer, from inside the Quartz Composer application, or can be embedded into a Cocoa or Carbon application via supplied user interfacewidgets. While Quartz Composer is included with the iPhone SDK, as of December 2015[update] there is no way of running Quartz Compositions on iOS devices.

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Quartz programming through Quartz Composer works by implementing and connecting patches.[3] Similar to routines in traditional programming languages, patches are base processing units. They execute and produce a result. For better performance, patch execution follows a lazy evaluation approach, meaning that patches are only executed when their output is needed. There are three types of patches: Consumers, Processors, and External Input patches that can receive and output mouse clicks, scrolls, and movements; MIDI and audio; keyboard; or other movements. A collection of patches can be melded into one, called a macro. Macros can be nested and their subroutines also edited.

To control the order of rendering, each renderer is assigned a layer, indicated in its upper-right corner. Layers are rendered sequentially, lowest to highest. Renderers can be enabled or disabled, essentially turning on or off that particular layer. Turning off unused layers often results in better performance, since fewer upstream patches need to be evaluated.

Some patches can have subpatches, which allows for global parameter changes to just the included subpatches. This is useful for lighting, 3D transformation, and GLSL shaders, among other things. Subpatch support is indicated by square corners on a patch, rather than the typical rounded corners.

With Version 3.0, it became possible to turn compositions into Virtual Patches. These allow the user to reuse functionality without having to store duplicate copies in each composition. The Quartz Composer Editor allows the user to save a "flattened" copy (with the virtual patches fully expanded inside), for easy distribution. Version 4.0 extended this functionality even more, and automatically includes "flattened" copies of virtual patches for use as a fallback if the desired virtual patch isn't installed on the host system. This greatly simplifies composition distribution.

Network functionality was greatly improved with the release of Leopard. It became possible to transmit data and synchronize over a network interface, and it also added support for Open Sound Control transmission and reception.

Also new in Version 3.0 was the possibility to write custom patch plugins, using an Xcode template, and the notion of a "safe mode", where plugins and other unsafe patches fail to load. This prevents malicious compositions from performing dangerous or insecure operations. Custom patches using Apple's Xcode template are always considered unsafe.

It was possible to develop custom patch plugins for Version 2.0, but the API was undocumented and private, and was never supported by Apple. Eventually, templates were released to simplify this procedure.[4]

In the Quartz Composer editor, holding the option key while selecting "Preferences..." from the menu adds 3 additional tabs of options for the user to configure. These options include System settings, Editor settings, and QuickTime integration settings. Notable options include expanded tooltips, software rendering, and uncapped framerate rendering. Multisample antialiasing (MSAA) was added as a hidden option in version 4.0, allowing for antialiasing inside the QC Editor, though it only works on GPU's that support MSAA.

Data can usually be converted to other types transparently. In Quartz Composer 3.0, the connections between patches change color to indicate conversions that are taking place. Yellow connections mean no conversion is taking place, Orange indicates a possible loss of data from conversion (Number to Index), and Red indicates a severe conversion; Image to Boolean, for example.

Quartz Composer documents are called Compositions. Compositions are Binary Property Lists (though XML versions are also supported) with a filename extension.qtz, and a com.apple.quartz-composer-compositionUTI.[5] Patches, their connections, and their input port states are saved in the composition file. Images can be stored inside a composition as well, making for self-contained compositions with embedded graphics. By dragging a movie file into the Quartz Composer editor, a reference to the movie file is created, providing a changing image that can be connected to a renderer.

Compositions also store metadata such as composition author, copyright, and description. The user can also add arbitrary metadata items, if desired.

Many image formats are supported, including JPEG, JPEG2000, GIF, PNG, TIFF, TGA, OpenEXR, BMP, ICO, PDF, PICT, ICNS, and some raw digital camera types.[6] Images are maintained in their native form for as long as possible before rasterizing for display. This means that Quartz Composer will keep vector images as vectors when cropping, scaling, rotating, or translating which allows it to work with very large logical image dimensions without consuming large amounts of memory or processing time. Such functionality is most apparent when working with text-based images, or PDFs.

Version 3.0 added the ability to add annotations to areas of the composition, called notes. These notes parallel comments in other programming languages. Notes can be yellow, red, green, blue, or gray, and can overlap other notes.

In Version 3.0, the concept of Composition Protocols was introduced. Protocols provide a template of required and optional inputs and outputs to qualify conforming compositions for various purposes. The following protocols are available by default:

Graphic Animation - These don't have required inputs or outputs, but are required to render to the screen. Graphic Animations are useful for animated backgrounds in applications such as Keynote.

Image Filter - Modifies an image using filters. No renderers are allowed in Image Filters, unless they are inside of a Render In Image environment.

Graphic Transition - Generates a transition from a source image to a destination image over a fixed time interval.

In addition to protocols, compositions can also conform to different runtimes where Quartz Composer is available. In Leopard, there are runtimes for Tiger (32-bit), as well as 32-bit and 64-bit versions of the Leopard Quartz Composer runtime. The editor can also indicate used patches that are unsafe, or unavailable in Tiger to aid in making compatible compositions.

A System-wide Composition Repository is available as of Version 3.0.[8] This allows applications to share and make use of common compositions for effects and processing. It is also possible for applications to query the repository for compositions that match certain criteria, such as protocol conformance.

Quartz Composer has many similarities to Max/MSP or Vvvv although its primary usage is for graphical rather than audio processing. The ability to construct interactive video compositions that react to audio or MIDI signals but which can be played from any QuickTime-aware application has caused a great deal of interest in Quartz Composer from VJs.

A developer tool called Quartz Composer Visualizer was released with Quartz Composer 3.0 that allows compositions to be rendered across multiple screens on a single machine, or even spanned across several machines and displays.